You can reroute and keep AC and defrosters, but you can figure that out on your own depending on how you want to do it.
I am also sticking with the S13 doorpanels, the s14 ones are too wide and would need to be trimmed reuppolstered.
There is a gap between the door panel and dash, but that is where the s14 AC dash ports for s14 door panels are. So out comes AC from the AC ports, and the s13 door panel actually is curved right there and directs the air right onto me.
This swap can be done using basic hand tools and a cutting wheel. It also utilizes the S13 speedo cluster and mounting
brackets to keep it simple as possible.
First your going to need to get rid of that cracked up s13 dash and make sure for your sleek s14 new new.
Drop your streering column down from the dash support bar by unbolting the bolts underneath of it accordingly.
Unbolt remove the glovebox and unbolt the rest of the dash and ac/radio area stuff.
Remove the smaller portions of the AC tubing around the dash bar if you did not take that out with the dash.
Now you are going to want to get your cutting wheel, gloves, and safety glasses out and trim or delete this bracket on the far driver side of support bar, as it gets in the way of the 14 dash. I trimmed it and slightly bent it inwards to help with clearance.
Towards the glove box area you are going to want to remove this bracket from the dash support bar. It is secured by two 10mm nuts and easily removeable.
The driver side bottom dash bracket needs to be modified to position it with the hole on the s14 dash. It is bolted on by two 10mm and only needs to move a small amount. You can slot the existing holes in the brackets to add adjustment and fit it accordingly.
The passenger bottom side dash bracket would be more difficult to reposition and is fairly close, so I just put some muscle into
positioning the corner of the dash inward and used a longer bolt to set it in place.
Now your going to want to prep your S14 dash. Unscrew the AC ducting and remove it and what ever else might still be on there till its pretty bare like below. We will be using the two bolt holes in the defroster area of the s14 dash to attach it to the s13 support bar. The rest of the plastic mounting pegs and mounting brackets for the ac ducts and stuff must be trimmed off along the back edge to get fitment to the firewall as close as possible.
Afterwards you need to trim the back of the dash about a cm or so. I used the cutting wheel and followed the contoured line near the edge and cut right on the outside of that leaving a slightly shorter cut on the 6" segment near the side of dash. Fitment is almost perfect and there is no noticable gaps when installed. I was able to fit the trim back between the dash and a-pillar aswell even though it was a tight fit.
Next you are going to need to trim the plastic in the cluster area on each side to be able to fit the s13 cluster in there. It all gets covered up by the speedo bezel, but if you make it a tight fit it will help support the speedo in the dash.
The cluster is screwed into place using only the top bracket but along with tight squeeze in the dash slots I made its good enough
and doesnt vibrate. The bottom two mounting tabs of the s13 cluster did not line up with s14 dash and got in the way so they got the chop.
When completed its a pretty flush fit and you can only see two tiny gaps when you look upwards at it from below. I also popped the s14 dimmer switch out and snapped the s13 one in its place.
The cops or inspection people might look at you funny for this one.
Once you have achieved the above, it is time to install the dash. It is easiest to put the driver side in threw the passenger door
and over the dropped steering column. Bolt it up in the four locations you have made available by modifying and trimming.
The clips for the S13 and S14 hazard and defrosters are plug and play so just unplug the s13 and clip them in when you install the s14 bezel.
Now would be a good time to install your glovebox and accessories onto the dash. After that you will be ready to attach the center console.
Now you can ball hard with your minty fresh s14 dash.
I was taking pictures while removing everything or else I would have completed product. This will have to do for now and ill update when its painted. 
